In the Universal Mobile Telecommunications System (UMTS) communications between a mobile station, e.g. a cellular telephone, and a base station require both an uplink channel (from the mobile to the base station) and a downlink channel (from the base station to the mobile). It is usual for the channels to be provided either as a Time Division Duplexing (TDD) or a Frequency Division Duplexing system. Often different operators within the same coverage area will have one or both systems available for their customers.
In TDD, a particular frequency channel is divided into time frames, with each frame being subdivided into a plurality of timeslots. Some of the timeslots in each frame are designated for uplinking and some are designated for downlinking, with a particular mobile being allocated particular uplink and downlink timeslots for a particular communication session. Of course, different operators will, in general, have different frequency channels allocated to them.
In FDD, a number of frequencies are available as communication channels and, for a particular communication session with a mobile, the operator will allocate one frequency channel as an uplink channel and one frequency channel as a downlink channel. Those particular frequency channels then remain available all the time for that mobile for the duration of the communication session. Of course, different mobiles may well share the same uplink and downlink channels using different codes.
When a mobile station operating in FDD mode moves about a particular base station coverage area, it is controlled to increase or decrease its transmission power depending on how close to the base station it is at any particular time. It will thus be apparent that if the mobile is at a relatively far distance from the base station, its transmission power will be high. If it should happen that the mobile is close to a base station operating in TDD mode, then any frequency channels available for the TDD communication that are adjacent the FDD uplink channel that the mobile is communicating on will have interference from the FDD mobile due to power leaking across adjacent frequencies from the FDD uplink channel. Such leakage can desensitise the TDD base station by causing it to reduce its reception sensitivity in the TDD frequencies adjacent the FDD uplink channel frequency so that it can no longer adequately receive TDD uplink communication at those frequencies.
This problem can occur even when the FDD mobile is close to the FDD base station if that happens to be co-located or closely located to the TDD base station and the FDD uplink channel has a broad transmission frequency spectrum. One way to overcome the problem is to use very sharp RF filters to preclude or reduce any crossover from the FDD uplink channel to adjacent channels, but such filters can be costly.